All living organisms mutate. Understanding the biology of any organism, past and future, requires understanding the mutational process. The ultimate objective of the proposed research is to understand the genetic and environmental factors underlying variation in the mutation rate. Those factors potentially include properties intrinsic to particular species and individuals. The specific aims of this proposal are: (1) characterize within- and between-species variation in the rate and molecular spectrum of spontaneous mutations, (2) determine if the genomic mutation rate has evolved to different optimal values in different organisms, and (3) experimentally characterize the relationship between metabolic rate and mutation rate. To achieve aim (1) existing stocks of Rhabditid nematodes that have accumulated mutations in the absence of natural selection for 250 generations will be sequenced at 100Kb of random genomic DNA. To achieve aim (2) a similar mutation accumulation experiment will be performed using historically outcrossing nematodes to contrast to self-fertile nematodes. Significant differences between selfing and outcrossing taxa in the rate and/or effects of new mutations will provide evidence that the genomic mutation rate is subject to "fine-tuning" by natural selection. Aim (3) is motivated by the apparent positive correlation between metabolic rate and rate of evolution. The relationship is plausibly attributed to DNA damage caused by oxygen radicals generated as by-products of metabolism, but several alternative explanations exist. Evidence suggests that any two randomly chosen worms may vary by as much as ten-fold in their mutation rate, and that the underlying molecular mutation rate may actually be ten times higher than is generally thought. If those results prove general, there would be important consequences fort the understanding of how mutations occur in humans, and on the nature of disease caused by complex genetic factors. Perhaps of even more importance, the proposed work provides a critical validation of the methods currently used to infer mutation rate in any organism.